Ceramics are used extensively in a large number of industrial applications. They are used as building materials, as cements and mortars, as abrasives, and in recent years ceramics have been developed for specialized uses in such fields as electronics, communications, and medicine.
In medicine, biodegradable macroporous ceramic scaffolds have been used as engineered grafts for tissue engineering, particularly bone tissue engineering. Such scaffolds typically are made with hydroxyapatite (HA) or tricalcium phosphate (TCP), or a combination of HA and TCP, with additives such as silica, magnesium, sodium, potassium, and zinc. The porous nature of these scaffolds permits the ingrowth of vascular and structural tissues and, because the scaffolds are biodegradable, can be used safely and without the need to remove the implant from the body.
For bone repair, particularly for defects in the spine and long bones, such as the bones of the legs, it is critically important that a ceramic scaffold implant have a high compressive strength and that this strength is maintained as the implant is biodegraded before the bone itself has healed and has sufficient strength. However, there is an inverse relationship between porosity and mechanical strength of the implants as the mechanical strength decreases as the porosity and pore size increases. In addition, biodegradable synthetic bone implants decrease in strength as the implant is degraded by contact with body fluids. Loss of strength of an implant at a time before the healed bone is able to support weight or support itself can lead to failure of the implant and of the repair process.
Ma, U.S. Pat. No. 6,673,285 discloses a method for fabrication of porous articles, such as polymer scaffolds. Ma discloses that the scaffolds may be made by casting a composition onto a negative replica of a desired macroporous architecture of the porous article to form a body, and that the negative replica, referred to as a porogen, is removed, thereby forming the porous article. Ma discloses that this method may be utilized to form a porous article from various materials, including polymers, ceramics, glass, and inorganic compounds.
The inventors have utilized the method of Ma in order to attempt to make macroporous ceramic calcium phosphate (CaP) scaffolds. Such attempts, however, were unsuccessful and this process could not be used to form a sintered integrated ceramic body. It was found that the ceramic article produced in this manner lacked sufficient hardness and strength and broke into a multiplicity of pieces before and during sintering.
Various scientific articles describe methods of manufacture of macroporous ceramic (CaP) scaffolds of various porosity and report on the compressive strength of these scaffolds. See, Hing, J. Mater. Sci. Mater. Med., 10(3):135-145 (1999); Liu, Ceramics International, 23:135-139 (1997); Seplveda, J. Biomed. Mater. Res., 50:27-34 (2000); Ramay, Biomaterials, 24:3293-3302 (2003); Almirall, Biomaterials, 25:3671-3680 (2004); Cyster, Biomaterials, 26:697-702 (2005); Silva, Biomaterials, 27:5909-5917 (2006); Uemura, Biomaterials, 24:2277-2286 (2003); Sous, Biomaterials, 19:2147-2153 (1998); Guo, Tissue Engineering, 10:1830-1840 (2004); Kwon, J. Am. Ceramic Soc., 85:3129-3131 (2002); and Milosevski, Ceramics International, 25:693-696 (1999). These reports show that the strength of porous CaP scaffolds tends to decrease with increasing porosity and that most of the scaffolds produced by the prior art methods have a compressive strength of only about 0.8 to 8 MPa (megapascals) with one report of a scaffold having 70% porosity, pores not completely interconnected, and a compressive strength of about 11 Mpa.
A significant need exists for a method by which the compressive strength of ceramic articles may be increased. Such a need is critical for ceramic articles, such as synthetic bone grafts, that are intended to be weight bearing, and is especially critical for making ceramic articles that have intercommunicating pores throughout their structures.